Cutting tool wear monitor

ABSTRACT

A method and apparatus by which the degree of wear and useful life limitations of a drill, end mill or other types of metal removal tools can be detected. The method is based on the short circuit current, open circuit voltage and/or power that is generated during metal removal by the utilization of an insulated rotary tool bit to which electrical contact is made by a non-rotating conductor and an insulated or non-insulated workpiece, with an external circuit connecting the tool and workpiece through a measuring device. The generated current, voltage or power shows a sharp increase or change in slope upon considerable tool wear and/or at the point of failure.

The present application is a continuation-in-part application of U.S.patent application Ser. No. 621,834, filed June 18, 1984, and nowabandoned.

BACKGROUND OF THE INVENTION

Tool wear has an important bearing on the performance of metal removaloperations, where worn tools may result in scrapped workpieces due tounacceptable surface finish, out of tolerance dimensions, or damagecaused by tool breakage. For these reasons, it has become commonpractice in machining operations to replace a cutting tool long beforethe end of its useful life, resulting in poor utilization of tools.Thus, there is the need for an effective method of measuring the amountof wear on a tool while cutting is in operation. At the present time, anacceptable tool wear sensing technology does not appear to exist.

Technologies have been and are being explored for monitoring cuttingtool wear based on fundamental features and phenomena of wear andfailure mechanisms that have been observed during cutting operations.Briefly, three distinct failure mechanisms have been identified:

1. Gross plastic deformation caused by excessively elevatedtemperatures.

2. Fatigue caused by excessively large cutting forces.

3. Gradual wear caused by the processes of adhesion, abrasion,electrochemical conversion and atomic diffusion.

The wear and failure mechanism that occurs in a specific situationdepends on the cutting forces, temperature, and the tool and workpiecematerials (e.g. composition, grain structure, surface composition). Thevariety of wear and failure mechanisms have resulted in various modes oftesting and monitoring.

One such measurement involves the dimensional changes of the cuttingtool or workpiece. This class of techniques includes mechanical gauging,profile tracers, weighing, ultrasonics, optical comparator methods andradiotracer methods. Except for radiotracer methods, all of thesetechniques are off-line measurements that frequently miss detection ofthe approach to failure. Also, radioactive methods are slow andperceived as unsafe.

The relationship between cutting forces and tool wear have been activelyexplored over the last twenty-five years, but a general correlation hasnot been established. For example, progressive flank wear producesincreasing forces whereas progressive crater wear has the oppositeeffect. The observed forces also depend on material hardness, depth ofcut and cutting speed. These techniques are hard to implement; requiringcareful placement of strain gauges or dynomometers. Transducersfrequently have to be incorporated into the original design of themachine.

Measurement of power consumption by the spindle or feed motor of amachining tool is easy to implement on new or existing machines. Suchtechniques have the potential of providing real-time optimization ofmetal removal rates, but serious disadvantages are inherent in thesemethods. Wear produces very small changes in power consumption whichmust be detected as a perturbation of a much larger signal (e.g. theoverall power consumption of the motor). These methods are sensitive tonon-wear related factors. Progressive wear of the tool increases powerconsumption but plastic deformation of the tool at high temperaturedecreases power consumption.

The bulk temperature of a tool can be measured by an embeddedthermocouple or infrared technique. Infrared measurement has thedisadvantage of requiring a very clean environment not found on amachine production floor, while embedded thermocouples require extensiveredesign of a machine's spindle. The rise in the bulk temperature of thetool, caused by wear, is very small and the signal to noise ratio ispoor.

Currently, vibration and sound analysis are active research areas,possibly because of the successful application of these methods to thestudy of rotating machinery and crack propagation in ceramics. Theresults of such analysis are very specific to a particular set-up andhard to generalize, and the techniques are difficult to implement,expensive and difficult to utilize. Therefore, a number of differenttechniques have been tried over the years the little success in thepredictability of excess tool wear and/or failure. The present inventionovercomes the deficiencies of previous techniques to provide aneffective tool wear monitoring technique.

SUMMARY OF THE INVENTION

The present invention relates to a novel method for monitoring wear of arotating cutting tool based on short circuit current, open circuitvoltage and/or power generated during the cutting operation on aworkpiece. As the tool wears, the generated current, voltage or powergradually increases until a generally sharp increase signifies failureof the tool due to excessive wear or breakage.

The present invention also relates to the provision of an apparatus formonitoring tool wear in drilling, milling or other machining or metalremoval operations wherein the electrical resistance along a path fromthe tool to the measuring circuit input for the workpiece must be muchless than any other potential electrical path. Depending on the machinetool, both tool and workpiece might need to be insulated, neither mightrequire insulation, or one, but not both, might require insulation. Anon-rotating contact for the tool can be utilized where it provides lowelectrical resistance.

The present invention further relates to a method of predicting toolfailure for a metal removal tool based on the material being worked on,the elapsed time of use of the tool and the current being generated bythe metal removal operation. Tool failure may be detected by acomparison of the observed generated current to the predicted currentfor failure or based on the derivative of the observed current, i.e. theslope change in a graph of generated current vs. time. Also, a lowimpedance current measurement is utilized to reduce error in the systemfrom utilization of the current to provide a reading thereof.

Further objects are to provide a construction of maximum simplicity,efficiency, economy and ease of assembly and operation, and such furtherobjects, advantages, and capabilities as will later more fully appearand are inherently possessed thereby.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a combined schematic and sectional showing of the cutting toolwear monitoring apparatus applied to a drilling machine tool.

FIG. 2 is a graph showing generated current versus the number of holesdrilled for a dry high speed tool steel bit.

FIG. 3 is a graph similar to FIG. 2 except for a dry oxide coated highspeed tool steel bit.

FIG. 4 is a graph similar to FIG. 2, but for large hole drilling.

FIG. 5 is a graph similar to FIG. 2, but for deep hole drilling.

FIG. 6 is a graph illustrating average current versus time of millingfor the tool in an end milling operation.

FIG. 7 is a graph similar to FIG. 6, but showing average voltage versustime of milling.

FIG. 8 is a graph illustrating a further milling operation plotting theaverage current output generated during metal removal versus elapsedtime.

FIG. 9 is a showing of a circuit for low impedence current magnitudemeasurement for a cutting tool monitor.

FIG. 10 is a showing of a circuit for low impedence current timederivative measurement.

FIG. 11 is a showing of the circuit of FIG. 10 with an additionalcontrol feature.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring more particularly to the disclosure in the drawings wherein isshown an illustrative embodiment of the present invention, FIG. 1discloses a machine tool 10 utilized for the drilling of holes in aworkpiece 11 wherein the machine includes a machine head 12 having arotating collet 13 mounted therein for rotation by a motor (not shown).A collet insert or drill holder 14 may have a shrink plastic tubingthereon to insulate the holder from the collet 13; a set screw 15 andinsulator 16 being mounted in the collet to further secure the holdertherein. A drill 17 is received in a blind passage 18 in the lower end19 of the holder and a set screw 21 retains the drill therein forrotation by the machine. An insulating washer 22 encompasses the holder14 between the end of the collet 13 and a shoulder 23 on the holder.

Mounted on the machine head 12 is a platform 24 having a dependingbracket 26 secured thereto and insulated therefrom by an insulator 25. Ahorizontal arm 27 on the bracket 26 extends laterally to the axis of theholder 14 and has an opening 28 therein through which the drill holderprojects. An annular phenolic brush holder 29 is secured to the arm 27by retaining screws 31 and has a central opening 32 axially aligned withthe opening 28 in the arm. The holder 29 has a plurality ofcircumferentially spaced radially extending passages 33, each passagereceiving a brush assembly 34 that is in electrical contact with thelower portion 19 of the drill holder 14. Each passage 33 has an enlargedthreaded outer portion 35 to receive a nylon brush retainer screw 36. Alead 37 extends through a central opening 38 in each screw 36 and isconnected to the outer end of the brush assembly 34, the opposite end ofeach lead terminating in a brush connector ring 39.

All of the connector rings 39 are connected to a lead 41, and a secondlead 42 is connected to the workpiece; with the leads 41 and 42 beingconnected to a suitable measuring device 43 having a circuit that willprovide measurement of voltage, current and/or power. This device wouldalso have predictive functions from a generated signal. The device 43receiving the leads 41,42 includes an indicator dial 51 showing thelevel of the generated current, voltage and/or power and a knob 52 isprovided to adjust the level of current at which a signal will be passedthrough leads 53,54 to an indicating means 55 to indicate that the toolhas reached a threshold voltage, current or power condition with achange in slope indicating approaching failure. The means 55 is shown asa speaker to provide an audible signal to the machine operator, althougha visual signal, such as a signal light would also be appropriate. Inthe alternative, the signal that is emitted from the device 43 couldresult in an automatic shut-down of the tool without intervention by theoperator.

For the predictive functions, a means for monitoring the machining timeis included in the device 43. The method of measuring the current and/orvoltage may also be important. For instances that produce uniform wearof all cutting edges of the tool, a measurement of the average voltageand/or current is sufficient. However, where uniform wear is notproduced on all of the cutting edges, average measurements may not besufficient, and measurement of the voltage and/or current generated byeach cutting edge may be required. This could be performed by very rapiddata acquisition that is synchronized with the rotational frequency ofthe edges. An easier alternative might be the rapid collection of a setof non-synchronized measurements; wherein some measurements in the setwill have contributions from more than one cutting edge, but some of themeasurements will have a predominant contribution from a single edge. Asincreased wear generates larger voltages and currents, the largest valuein the set of measurements, therefore, should have come from the mostworn edge.

Although the tool 17 and workpiece 11 are shown as requiring insulation,the important factor to be considered is that the electrical resistancealong a path from the tool 17 to the measuring circuit input to theworkpiece must be less (e.g. 100 or more times less) than any otherpotential electrical path. Depending upon the machine tool, especiallythe electrical resistance of the spindle bearings, both tool andworkpiece may need to be insulated, neither may need insulation, or one,but not both, may need to be insulated. Obviously, it is preferable notto have to insulate either one.

The design of the rotating contact for the tool or tool holder requiresonly that it provides low electrical resistance and little reduction ofthe rigidity of the tool/tool holder system. Although shown as aplurality of brushes 34 in a non-rotating ring or holder 29, a mercuryslip ring or other suitable structure could also be used.

A characteristic curve for a drilling operation is shown in the graph ofFIG. 2 where the generated current is plotted versus the number of holesdrilled by a high speed tool steel bit in the dry drilling of 01 toolsteel workpieces. The curve 45 has an initial steep slope A which thengenerally levels off in area B and finally reaches a steep incline C atthe point of threshold current indicating near tool failure. The drillhad a 0.25 inch bit rotated at a speed of 1150 rpm with a feed of 0.006inch per revolution and a depth of 0.375 inch. The current amplitude atthe onset of failure increases by 50 to 100% compared to the averagecurrent level over the B area of the graph and may occur over anextremely short period of time.

FIG. 3 shows an illustrative graph for the dry drilling of an 01 toolsteel workpiece with an oxide coated high speed tool steel bit. Hereagain, an initial steep incline A' of curve 46 is followed by agenerally lateral line B' and finally a sharp incline C' to falure.Similar results are obtained in dry drilling of steel utilizing an oxidecoated tool steel bit, and similar results occur with the drilling ofstainless steel. A lubricated cutter tool works as well as a dry cuttingtool for the monitoring operation.

FIG. 4 illustrates a graph for a large hole drilling operation using a0.75 inch high speed tool steel bit on a 01 tool steel workpiece at 800rpm for a hole depth of 0.375 inch with a feed of 0.003 inch perrevolution. Here again there is an initial steep incline A" of curve 47,a lateral stepped area B" and a final steep incline C" to approximatetool failure.

FIG. 5 shows a graph illustrating generated current versus the number ofholes drilled to a constant depth for deep hole drilling. A 0.25 inchoxide coated tool bit is rotated 800 rpm and fed at 0.003 inch perrevolution acting on a 01 tool steel workpiece for a depth of 2.0 inch.As seen the initial incline A"' of curve 48 and final incline C"' arenot as steep as those for shallower holes, but the final incline appearsindicative of a threshold current predicatable of tool failure.

All of the previous graphs illustrate the indication of catastrophictool failure by a slope change of the threshold current, however, aspecific degree of wear can be detected by comparison of the observedcurrent, voltage or power to a reference value. Furthermore, theremaining life of the tool can be predicted from the observed current,voltage or power by use of a reference equation that relates theobserved signal to the machining time. The form of this equation istypically ##EQU1## where I is the observed current, V is the voltage, Pis the power, t is the machining time at which the electrical parameterwas observed, t failure is the predicted time of tool failure and a andb are constants that depend on the nature of the machining operation.

FIG. 6 illustrates a characteristic curve 49 for a milling operationutilizing a 0.25 inch high speed tool steel cutter or bit on a 01 toolsteel workpiece where the bit operates at 600 rpm with a feed of 0.5inches per minute and a 0.03 inch depth of cut. Both drilling andmilling operations are typically performed on the same or similarmachines, with the primary difference residing in the geometry of thetool cutting edges; i.e., the depth and height of the cutting flutes.The curve 50 superimposed over the curve 49 for the current generated isderived from the following general equation: ##EQU2## which equation isa more general form of the first equation for predicting tool failure.The curve 49 generally follows the curve derived from the last equationuntil a larger deviation is noted when the wear of the tool becomesgreater and approaches tool failure.

FIG. 7 is a similar curve 56 where the voltage is plotted against theelapsed time of the milling operation. This curve 56 generallycorresponds to the curve 49 in FIG. 6.

FIG. 8 is a third curve 61 where average current is plotted against theelapsed time of a milling operation and shows a sudden slope change at62 indicative of tool wear at approximate failure condition. The smoothline 63 is indicative of the calculated current utilizing the firstequation on page 10.

To provide a low impedence current measurement for the current generatedduring the metal removal operation, a circuit is illustrated in FIG. 9where an operational amplifier (integrated circuit) 65 has an input line66 to the minus side for the generated current from the tool of FIG. 1and the plus side is connected to ground 67. Likewise, the workpiece 11engaged by the tool is grounded. Line 69 has a current booster 71 formedof a pair of transistors to boost the current sourcing capability of theamplifier 65. A feedback loop 73 containing a suitable resistance 74extends from line 72 after the booster 71 to return to line 66. Thevoltage output to ground 76 across point 75 is proportional to thecurrent input at line 66.

The amplifier 65 acts to balance the current input with the currentfeedback through loop 73 so the current difference at point 68 is zero.To provide a current balance, some voltage must be generated with thisvoltage output indicating the current input without actually using anycurrent in the system to avoid error. Because of the zero current, thiscircuit provides a low impedence measurement for the tool.

Similarly, the circuit of FIG. 10 is supplemental to the circuit of FIG.9 with like parts having the same numeral with a script "a". Theoperational amplifier 65a receives the current input from the toolthrough line 66a with the output boosted by the current booster 71a inline 69a. The feedback loop 73a containing resistance 74a leads fromline 72a back to point 68a. The line 72a connects to a line 77containing a variable capacitor 78 and leading from the output of thebooster 71a to the minus side of a second operational amplifier 79; thepositive side being grounded at 81. A feedback loop 83 from output 82contains a resistance 84 and capacitor 85 in parallel and extends fromoutput 82 on the output side of amplifier 79 to the line 72a. Thevoltage output at line 82 is proportion to the derivative of the currentinput with respect to time; i.e. the time rate of change of the currentor slope.

The circuit of FIG. 10 is shown in FIG. 11 with the addition of acomparator 87, with like parts having the same numeral with a script"b". In this embodiment, the voltage output from line 82b is connectedto the negative side 88 of a third operational amplifier 87 orcomparator. The positive side 89 of the comparator is connected to areference voltage corresponding to the slope change to be detected. Theoutput 91 from the comparator provides an on-off signal to a relay forthe feed motor and/or alarm for the cutting system. Obviously, thecomparator 87 may be added to the circuit of FIG. 9 with the negativeside 88 connected to the voltage output of line 72.

Although shown and described for use in a drilling or milling operationof metal removal, it is contemplated that the tool monitoring andfailure predicting function could also be effectively utilized in othermetal cutting or removing operations such as turning on a lathe orshaping, and all such metal removal operations are contemplated in thefollowing claims.

We claim:
 1. A method of detecting tool wear failure for a metal removal operation comprising the steps of monitoring the direct current generated by a cutting tool acting on a metal workpiece during the cutting operation and determining a sharp change in the slope of the current generated plotted against the number of articles cut in comparison with earlier changes in slope thereof as an indication of tool failure.
 2. A method of detecting tool wear failure as set forth in claim 1, wherein tool failure occurs upon a 50 to 100% increase in generated current over the average current during the cutting life of the tool.
 3. A method of detecting tool wear failure as set forth in claim 1, wherein failure of said tool is predicted according to the following equation: ##EQU3## wherein I is the generated current, t is the elapsed time of cutting for the tool, t failure is the time elapsed for failure and a and b are constants depending on the nature of the tool, workpiece and machining operation.
 4. A method of detecting tool wear failure as set forth in claim 1, wherein the current measurement is performed at a low impedence to reduce error.
 5. A method of detecting tool wear failure as set forth in claim 1, wherein failure is detected by comparison of the observed current relative to the predicted current for the elapsed time of metal removal of the tool.
 6. A method of detecting tool wear failure as set forth in claim 1, wherein failure is detected on the basis of the derivative of the observed current with respect to time.
 7. A method of detecting tool wear failure for a metal removal operation comprising the steps of monitoring the direct current generated by a cutting tool acting on a metal workpiece during the cutting operation and determining a sharp change in the slope of the current generated plotted against the number of articles cut in comparison with earlier changes in slope thereof as an indication of tool failure, said current measurement being performed at a low impedence to reduce error, wherein a current balancing circuit receives the tool-workpiece generated current and provides a voltage indicative of the generated current.
 8. A method of detecting tool wear failure as set forth in claim 7, wherein said circuit includes an operational amplifier receiving the current input, a current booster from the ampifier output, and a feedback loop from the current booster to the current input from the tool.
 9. A method of detecting tool wear failure as set forth in claim 8, including a comparator receiving the voltage output from the current booster and providing an on/off control for the metal removal tool.
 10. A method of detecting tool wear failure for a metal removal operation comprising the steps of monitoring the direct current generated by a cutting tool acting on a metal workpiece during the cutting operation and the total elapsed time of cutting by the tool and predicting failure of the tool from the following equation: ##EQU4## wherein t failure is the elapsed time for tool failure in seconds,t is the elapsed cutting time for the tool in seconds, I is the observed current in microamperes, and a and b are constants depending on the machining operation.
 11. A method of detecting tool wear failure for a metal removal operation comprising the steps of monitoring the direct current generated by a cutting tool acting on a metal workpiece during the cutting operation and detecting failure on the basis of the derivative of the observed current with respect to time, wherein a low impedence circuit includes a first operational amplifier receiving the current input from the tool and providing current output, a current booster from the current output, a feedback loop for the amplified current to the current input lead, a second operational amplifier receiving a voltage output through a variable capacitor and providing a voltage output proportional to the derivative of the current input with respect to time.
 12. A method of detecting tool wear failure as set forth in claim 11, including a feedback loop from the voltage output to the voltage input, and a comparator receiving the voltage output to control the tool operation. 